1. Field of the Invention
The invention relates to a booth for the spray-coating of articles with powdery coating material, in a booth interior defined by walls, a booth floor and booth ceiling and having a specific longitudinal axis, in the direction of which the workpieces are movable in or through the booth, the booth being assigned a cleaning device which is movable in the booth interior in the direction of its longitudinal axis.
2. Description of Related Art
Booths for electrostatic powder coating are being used increasingly, particularly for reasons of greater environmental compatibility, such as is described in general terms in the VDMA Standard Sheet, 24 371, Part 1 of March 1980 and Part 2 of August 1983. In this, the coating material for the most diverse kinds of constructional or working parts or other articles of daily need is not in the form of a dispersion dissolved in a solvent, that is to say, is not for example put on by the paint-spraying method, but is applied in the form of a granulate. This granulate is, as a rule, conveyed by means of an injector out of a fluidizing vessel through a hose to a powder gun. Mounted in this gun are electrodes which are subjected to high voltage and which charge the powder electrostatically. The charged powder is sprayed by the gun in the direction of the grounded workpiece. The charged powder settles on the workpiece, specifically usually at approximately 40% of the quantity of sprayed material. The powder fraction sprayed past the workpiece is intercepted in the powder booth and returned to the coating process.
It can be seen from this that the main problems of this powder coating method are, on the one hand, to recover as completely as possible the powder not adhering to the article to be coated and, on the other hand, in the event of color changes, to bring about a cleaning of the booth interior which is sufficiently thorough, but above all as rapid as possible, so that the booth can be used again in a short time, that is to say to keep the change-over and therefore idle times as short as possible.
Pursuing the idea of working in a material-saving manner and, when powder-spraying booths are being used, quickly returning to the powder circuit as much as possible of the sprayed powder not adhering to the workpiece, a booth having the form of an upright hollow cylinder has become known (DE-A1-3,538,800). In this is arranged a single-wing or multi-wing ceiling, wall and floor scraper which rotates about the cylinder axis and by means of which excess powder is removed from the inner walls and fed via an inflow in the floor to a powder recovery device. A complete cleaning of the booth, for example in the event of color changes, is therefore impossible, because the scraper, including its driving elements, itself becomes soiled and requires manual cleaning. Furthermore, such continuously operated scrapers cause the powder to agglomerate on the wall and scrapers. Such agglomerates can be removed only by separate cleaning, for example with solvents, in order thereby to avoid the danger of inoperability. With the rotational arrangement, it is virtually impossible to remove excess powder in the central region.
To automate the time-consuming cleaning operation in the event of a color change and to shorten this, there has become known a booth of the type described in the introduction (German Offenlegungsschrift 3,516,826), in which a pipe-ring system movable along the booth axis is provided, this having nozzles through which compressed air is ejected in pulses, in order to swirl the excess powder adhering to the walls. This is then to be sucked away by the main suction, whilst, in order to clean the pipe-ring system itself, nozzles are additionally arranged in the booth end walls. This known cleaning system does not function satisfactorily, because some of the powder, above all comprising very fine fractions, wanders about in the booth space and settles indeterminately, particularly in the corner regions of the booth. This gives rise, in practice, to the need for subsequent cleaning, if appropriate by means of wet sponges, troughs or the like.
In another cleaning system known from practice, the booth walls are formed from a rollable plastic, usually polyethylene sheet. This sheet is kept in stock on rolls which are further rotated in the event of a powder change. For safety reasons, such arrangements are completely inadmissible in many countries and otherwise, above all in view of the problems of sealing occurring between the sheet webs which define the walls and which moreover can be supported only with difficulty, can be employed merely to a limited extent. On the contrary, a principal requirement of spray-coating booths is that the booth housing should have stable walls, in order to avoid the problems of an incompletely sealable booth interior which arise as result of unrolled booth walls and which lead to difficulties in adhering to work-safety and fire protection regulations.
For all these reasons, therefore, manual cleaning methods are generally used today, as before, for booths with stable walls, and the powder adhering in the booth is removed by means of scrapers, sponges, damp cloths or suchlike aids.